1. Field of the Invention
The present invention relates to an image reading device in which, when an original image is read by a solid-state image sensor via a lens, a variable aperture mechanism, which is disposed in the vicinity of the lens and can change the amount of light to be adjusted, adjusts the amount of light in the photo-receivable range of the solid-state image sensor.
2. Description of the Related Art
In an image reading optical system in which light is illuminated onto an original image, the reflected or transmitted light is guided to a CCD sensor via a lens, and the image is picked up, an aperture mechanism is disposed on an optical path so as to adjust the amount of light which to reach the CCD sensor.
Further, in case of a color CCD sensor, a color balance filter which adjusts the balance of colors, an IR cut filter which cuts infrared rays, and the like are disposed in the vicinity of the lens. Thus, only the light which is necessary to read an image reaches the CCD sensor.
The above aperture mechanism adjusts the amount of light so that the area of the amount of the above reflected light or transmitted light falls within the dynamic range of the CCD sensor. When the amount of light is great, the amount of obstruction of the optical path is increased, and when the amount of light is small, the optical path is set at full aperture.
The amount of control of the aperture mechanism is determined by the brightness of the lens (the ratio of the amount of irradiated light to the amount of incident light). For example, when the amount of output light is small in comparison to the amount of light incident on the lens, i.e., when a dark lens is used, the amount of control by the aperture mechanism may be restricted due to diffraction limit.
With the aforementioned in view, an object of the present invention is to obtain an image reading device in which the width of the amount of light to be controlled can be increased without generating drawbacks such as light diffraction or the like.
The first aspect of the present invention is an image reading device in which, when an original image is read by a solid-state image sensor via a lens, a variable aperture mechanism, which is disposed in the vicinity of the lens and can change the amount of light to be adjusted, adjusts the amount of light in the photo-receivable range of the solid-state image sensor, wherein: in a state in which the variable aperture mechanism is stopped substantially in the center of the range in which the amount of light can be adjusted by the variable aperture mechanism, light reducing means is provided so that the output value of the solid-state image sensor corresponding to the maximum amount of light received by the solid-state image sensor is within a predetermined range.
In accordance with the first aspect of the present invention, if the amount of light reflected by the original image or the amount of light transmitted through the original image is large, it is possible that the amount of light is saturated and exceeds the photo-receivable range (dynamic range) of the solid-state image sensor. In this case, the variable aperture mechanism guides the light to the solid-state image sensor in a state in which the amount of light is reduced. At this time, if the amount of light is so-called stopped too much, the amount of light received by each pixel of the solid-state image sensor may be different to the actual original image due to light diffraction. Accordingly, the amount of light is reduced by a predetermined amount beforehand by the light reducing means, and thereafter, the amount of light is adjusted to the optimal level by the variable aperture mechanism.
In this way, it is no longer necessary to stop by the variable aperture mechanism by a large amount and drawbacks such as the light diffraction or the like can be prevented.
Further, even if the variation of the amount of light reflected by the original image or the amount of light transmitted therethrough is large, the light reducing means is provided substantially in the center of the range in which the amount of light received can be adjusted by the variable aperture mechanism, so that the value output by the solid-state image sensor, which corresponds to the maximum amount of light received by the solid-state image sensor, falls within a predetermined range. Consequently, the amount of light can be adjusted over a wide range without generating light diffraction.
Namely, the first invention achieves a superior effect in that the extent of the amount of light to be controlled can be increased without generating drawbacks such as light diffraction or the like.
A second aspect of the present invention is an image reading device in which, when an original image is read by a solid-state image sensor via a lens, a variable aperture mechanism, which is disposed in the vicinity of the lens and can change the amount of light to be adjusted, adjusts the amount of light in the photo-receivable range of the solid-state image sensor, wherein: a separate ND filter reduces the light by a predetermined amount so that diffraction is not generated due to the stop adjustment by the variable aperture mechanism for necessary adjusting the amount of light.
In accordance with the second aspect of the present invention, as the ND filter is used, the amount of light can be reduced without obstructing the optical path. When an ND filter is disposed on the optical path, if the ND filter is removable, an optimal ND filter can be selected for each of the devices. Moreover, if a plurality of ND filters are superposed, the ND filter can implement (gradual) control of the amount of light to a certain extent, and the variable aperture mechanism carries out only fine adjustment.
Namely, in the second aspect of the present invention, the amount of light can be reduced without obstructing the optical path.
A third aspect of the present invention is an image reading device in which, when an original image is read by a solid-state image sensor via a lens, a variable aperture mechanism, which is disposed in the vicinity of the lens and can change the amount of light to be adjusted, adjusts the amount of light in the photo-receivable range of the solid-state image sensor, wherein: a separate fixed aperture mechanism, in which the amount of light to be adjusted is fixed, reduces a predetermined amount of light so that diffraction is not generated due to the stop adjustment by the variable aperture mechanism for necessary adjusting the amount of light.
In accordance with the third aspect of the present invention, as the two aperture mechanisms (the variable aperture mechanism and the fixed aperture mechanism) are used, the amount of stop can be dispersed and drawbacks such as diffraction or the like can be prevented.
Namely, in the third aspect of the present invention, the amount of stop can be dispersed and drawbacks such as diffraction or the like can be prevented.